Uv photobleaching of glass having uv-induced colorization

ABSTRACT

A method of UV photobleaching a glass sample having UV-induced colorization is disclosed. The processed includes first irradiating the glass sample with colorizing UV radiation having a colorizing wavelength of λ C &lt;300 nm to form the colorized glass, which has a pink hue. The method then includes irradiating the colorized glass with bleaching UV radiation having a bleaching wavelength of λ B , wherein 248 nm≦λ B ≦365 nm, to substantially remove the pink hue.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 61/942,302 filed on Feb. 20, 2014the content of which is incorporated herein by reference in itsentirety.

FIELD

The present disclosure relates to photobleaching, and in particularrelates to UV photobleaching of glass that has UV-induced colorization.

BACKGROUND

Certain types of glasses are susceptible to colorization and reducedoptical transmission (i.e., higher optical absorbance) at visiblewavelengths when subjected to high-energy UV radiation. In particular, acolor center around 500 nm can arise wherein the glass takes on a pinkcolorization or hue due to increased absorption of blue-wavelength lightrelative to the red wavelengths. The UV-induced colorization can occurfor example when the glass is subjected to a process that includesrelatively high-intensity deep ultraviolet (DUV) radiation as the mainprocessing vehicle, or as a byproduct to different processing vehicle.For example, high-energy DUV radiation is present in substantial amountsas a byproduct in plasma cleaning processes and sputter depositionprocesses.

UV-induced glass colorization can be disadvantageous, especially inapplications where the glass is intended for use at visible wavelengthswhere substantially equal transmission over the visible wavelength rangeis required. Examples of such use in consumer electronics include usingtransparent glass sheets as cover glasses for various types of displays,which can range in size from a one or a few inches (e.g., cell phones,smart phones, etc.) to several feet (e.g., large-screen televisions).

SUMMARY

An aspect of the disclosure is a method of processing a glass sample.The method includes: irradiating the glass sample with colorizing UVradiation having a colorizing wavelength of λ_(C)<300 nm to form acolorized glass sample having a pink hue; and irradiating the colorizedglass sample with bleaching UV radiation having a bleaching wavelengthof λ_(B), wherein 248 nm≦λ_(B)≦365 nm, to substantially remove the pinkhue.

Another aspect of the disclosure is a method of processing a glasssample. The method includes: irradiating the glass sample withcolorizing UV radiation having a colorizing wavelength λ_(C)<300 nm,thereby forming a colorized glass sample having a pink hue due to theformation of an amount of Ti⁺³ color centers in the glass sample; andirradiating the colorized glass sample with bleaching UV radiationhaving a bleaching intensity I_(B) and a bleaching wavelength of λ_(B),wherein 248 nm≦λ_(B)≦365 nm, and wherein the bleaching intensity I_(B)is sufficient to substantially remove the pink hue by substantiallyreducing the amount of Ti⁺³ color centers in the glass sample.

Another aspect of the disclosure is a method of bleaching a colorizedglass sample that has a pink hue due to the formation of Ti⁺³ colorcenters. The method includes: irradiating the colorized glass samplewith bleaching UV radiation that has a bleaching wavelength λ_(B),wherein 248 nm≦λ_(B)≦365 nm; and carrying out said irradiating for anexposure time t in the range 0.5 hr≦t≦5 hrs to substantially remove thepink hue.

Additional features and advantages are set forth in the DetailedDescription that follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings. It is to be understood that both theforegoing general description and the following Detailed Description aremerely exemplary, and are intended to provide an overview or frameworkto understand the nature and character of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the Detailed Description serve to explain principles andoperation of the various embodiments. As such, the disclosure willbecome more fully understood from the following Detailed Description,taken in conjunction with the accompanying Figures, in which:

FIG. 1 is an elevated view of a glass sample shown by way of example inthe form of a glass sheet;

FIG. 2 is a side view of the glass sample of FIG. 1, wherein the glasssample is being irradiated with colorizing UV radiation;

FIG. 3 is a side view that shows the resulting colorized glass sampleformed by the irradiation of the glass sample with colorizing UVradiation as shown in FIG. 2;

FIG. 4A is a side view of the colorized glass sample being blanketirradiated with bleaching UV radiation to substantially remove the pinkhue;

FIG. 4B is similar to FIG. 4A, but shows an embodiment wherein thebleaching UV radiation is either scanned or stepped relative to thecolorized glass sample;

FIG. 5 is a side view of the glass sample after having been irradiatedwith bleaching UV radiation using the methods of either FIG. 4A or FIG.4B, wherein the pink hue has been substantially removed;

FIG. 6 is a plot of the optical absorbance A (mm⁻¹) versus wavelength λ(nm) for a glass sample made of an aluminophosphosilicate glass; and

FIG. 7 is a plot of the optical absorbance A (mm⁻¹) versus exposure timet (hrs) for bleaching UV radiation for an example colorized glass samplemade of an aluminophosphosilicate glass.

DETAILED DESCRIPTION

Reference is now made in detail to various embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Whenever possible, the same or like reference numbers andsymbols are used throughout the drawings to refer to the same or likeparts. The drawings are not necessarily to scale, and one skilled in theart will recognize where the drawings have been simplified to illustratethe key aspects of the disclosure.

The claims as set forth below are incorporated into and constitute partof this Detailed Description.

The entire disclosure of any publication or patent document mentionedherein is incorporated by reference.

Cartesian coordinates are shown in some of the Figures for the sake ofreference and are not intended to be limiting as to direction ororientation.

The vertical shading in the colorized glass sample shown in some of theFigures is used to represent the color pink (i.e., a pink hue) inaccordance with the shading guidelines for representing colors as setforth in the Manual of Patent Examining Procedures (MPEP), section608.02(IX).

The term “ppm/mole” of an element E stands for parts-per-million by moleof the oxide of E, and in the case where the element E is titanium, theoxide is TiO₂.

FIG. 1 is an elevated view of a glass sample 10 that has a body 12.Glass sample 10 is shown by way of example in the form of a sheetwherein body 12 includes generally planar and parallel upper and lowersurfaces 14 and 16. An example type of glass for glass sample 10 is analuminophosphate glass, such as disclosed in U.S. Pat. No. 3,746,556.Another example type of glass is an aluminophosphosilicate glass, suchas disclosed in U.S. Pat. No. 7,323,426. Another example type of glassfor glass sample 10 is an alkali aluminosilicate glass, such asdisclosed in U.S. Pre-Grant Publication No. 2013/0122284. In an example,glass sample 10 includes titanium, and further in an example thetitanium is tramp titanium, i.e., is present in glass body 12 in traceamounts as a contaminant. Thus, in an example, glass sample 10 can bemade of any type of glass that includes titanium in trace amounts.

FIG. 2 is a side view of glass sample 10, wherein the glass sample isbeing irradiated with colorizing UV radiation 20 from a colorizing UVradiation source (not shown). In an example, colorizing UV radiation 20has an intensity I_(C) and a colorizing wavelength λ_(C)<300 nm. In anexample, colorizing UV radiation 20 is a byproduct to the formation ofthe main vehicle for processing glass sample 10. For example, in plasmacleaning processes, the main vehicle for cleaning glass sample 10 is aplasma, while the UV colorizing radiation 20 is emitted by the plasma.Likewise, in a sputter deposition process, the main vehicle forsputtering material onto the glass sample 10 is an energetic beam ofatoms and ions, while the UV colorizing radiation 20 is a byproduct ofthe source of the energetic atoms and ions. In another example, the UVphotons of colorizing UV radiation 20 constitute the main vehicle of theprocess. Examples of such a process include optical inspection,annealing, decontamination, etc.

FIG. 3 shows glass sample 10 after being subject to colorizing UVradiation 20, wherein glass body 12 now has a pink colorization or hue.Glass sample 10 when so colorized is referred to hereinafter as“colorized glass sample 10.” As discussed in greater detail below, andwithout being bound by theory, it is believed that the pink colorizationor hue is induced in glass sample 10 by the photoreduction of traceamounts of titanium in glass body 12 caused by colorizing UV radiation20.

FIG. 4A is similar to FIG. 2 and shows colorized glass sample 10 beingblanket irradiated with a bleaching UV radiation 30 having a bleachingintensity I_(B) and a bleaching wavelength λ_(B). In an exampleembodiment, the bleaching wavelength λ_(B) is in the range 248nm≦_(B)≦365 nm. Bleaching UV radiation 30 originates from a bleaching UVradiation source 32. In an example, bleaching UV radiation source 32includes one or more optical components (not shown) that serve to directand/or shape bleaching UV radiation 30, e.g., to form a substantiallycollimated beam of bleaching UV radiation. In one example, bleaching UVradiation source 32 is a continuous wave (CW) source, while in anotherexample, it is pulsed source. The bleaching UV radiation source caninclude UV-emitting lasers, UV-emitting light-emitting diodes (LEDs),mercury-arc lamps, as well as a combination of these types of sources.

FIG. 4B is similar to FIG. 4A and shows an example wherein bleaching UVradiation 30 is scanned or stepped relative to colorized glass sample10, as indicated by arrow 34. In the scanning method, bleaching UVradiation 30 continuously moves relative to colorized glass sample 10.In the stepping method, bleaching UV radiation 30 resides at onelocation for an exposure time and then moves to another location. Inboth the scanning and stepping methods, the bleaching UV radiation 30may need to irradiate the same portion of colorized glass sample 10multiple times. While the scanning and stepping exposure methods maytake longer to substantially reduce or remove the pink hue, thebleaching UV radiation 30 can have a higher intensity I_(B) because theradiation beam can be made more concentrated. The scanning and steppingexposure methods illustrated in FIG. 4B may also be used to selectivelyexpose one or more portions of colorized glass sample 10.

FIG. 5 shows colorized glass sample 10 after being irradiated withbleaching UV radiation 30 such that substantially all of the pinkcolorization or hue has been removed from glass body 12.

In an example embodiment, bleaching UV radiation 30 can have arelatively low intensity I_(B), e.g., as low as 5 mW/cm². In an example,the intensity I_(B) of bleaching UV radiation is in the range 5mW/cm²≦I_(B)≦100 mW/cm². In another example, the intensity I_(B) ofbleaching UV radiation is in the range 10 mW/cm²≦I_(B)≦50 mW/cm².Exposure times t for bleaching UV radiation 30 depend on the intensityI_(B), but in an example can be in the range 0.5 hour≦t≦5 hours. Inexamples, some or all of colorized glass sample 10 is irradiated withbleaching UV radiation 30.

FIG. 6 is a plot of optical absorbance A (mm⁻¹) versus wavelength λ (nm)for a glass sample 10 made of an aluminophosphosilicate glass having acomposition (in mole %) of: SiO₂ (58), Al₂O₃ (16), P₂O₅ (6), Na₂O (17),MgO (3). The plot of FIG. 6 shows five curves C0 through C4. The dataassociated with curves C1 through C4 are based on measurements of theabsorbance taken for different exposure times for bleaching UV radiation30, wherein λ_(B)=254 nm and I_(B)=5 mW/cm². Curve C1 represents acolorized glass sample 10 that has not been irradiated with bleaching UVradiation 30 and so has the highest overall absorbance A over thevisible spectral range. Curve C2 represents the absorbance afterexposing colorized glass sample 10 to bleaching UV radiation 30 for anexposure time of t=2 hours. Curve C3 represents the absorbance afterexposing colorized glass sample 10 to bleaching UV radiation 30 for anexposure time of t=3 hours. Curve C4 represents the absorbance afterexposing colorized glass sample 10 to bleaching UV radiation 30 for anexposure time of t=3.5 hours. The absorbance curves C1 through C3 show apronounced absorption feature at λ=500 nm, which corresponds to bluevisible light. The absorbance of blue visible light gives colorizedglass sample 10 its pink colorization.

FIG. 6 also includes a curve C0 (dashed line) that corresponds to theabsorbance of glass sample 10 prior to be being colorized and bleached.Note how the bleaching process in this case actually reduces theabsorbance of glass sample 10 as compared to its original absorbance.

Colorization Mechanism

As noted above, without being bound by theory, it is believed that themechanism for the UV-induced pink colorization of colorized glass sample10 and the subsequent bleaching of the colorization is attributable tothe presence of trace amounts of titanium. In particular, it is believedthat the photoreduction of Ti⁺⁴ to Ti⁺³ in glass sample 10 leads to theincreased absorption of blue light that gives rise to the pinkcolorization or hue. In an example, the trace amounts of titanium (e.g.,as Ti⁺⁴ and Ti⁺³) are present in quantities in the range of about 20 to100 ppm/mole in glass sample 10 in its normal transparent state. Thetrace amounts of Ti⁺³ increase by up to about 10-fold (e.g., from 1 to 2to between 10 and 20 ppm/mole) when the glass is exposed to colorizingUV radiation 20. In experiments using electron paramagnetic resonance(EPR), the Ti⁺³ concentration in colorized glass sample 10 was measuredto increase about 10-fold relative to the original (uncolorized) glasssample.

In one example, the trace amounts of Ti⁺³ are present in quantities(amounts T) in the range 20 ppm/mole≦T≦100 ppm/mole in glass sample 10in its normal transparent state, while in another example, 20ppm/mole≦T≦1000 ppm/mole. The trace amounts of Ti⁺³ increase by up toabout 10× (e.g., to between 200 and 1000 ppm/mole) when the glass isexposed to colorizing UV radiation 20. In experiments using electronparamagnetic resonance (EPR), the Ti⁺³ concentration in colorized glasssample 10 was measured at 400 ppm/mole, while the original (uncolorized)glass sample had a Ti⁺³ concentration of about 40 ppm/mole.

Colorizing experiments were performed to examine which colorizingwavelengths λ_(C) and colorizing intensities I_(C) would give rise tothe pink colorization. Experiments showed that irradiation of glasssample 10 with UV radiation in the wavelength range from 254 nm to 360nm and at intensity levels from 10 mW/cm² to 20 mW/cm² did not causecolorization in the glass sample. On the other hand, experiments whereglass sample 10 was exposed for 10 minutes with colorizing UV radiationhaving a colorizing wavelength λ_(C)=248 nm from a pulsed KrF laser witha pulse rate of 20 Hz and an average power of 1.25 W/cm² caused theformation of the aforementioned pink colorization or hue.

In addition, colorizing experiments whereby glass sample 10 was exposedfor 16 minutes with colorizing UV radiation from a UV-based cleaner(namely, the UV-CLEANER® Model 7576 series from Jelight Company, Inc.,Irvine, Calif.) having a nominal colorizing wavelength of λ_(B)=254 nmwith an intensity between 28 mW/cm² and 32 mW/cm² caused the formationof the aforementioned pink colorization or hue.

In addition to the above-described colorizing experiments, it has beenobserved that glass samples 10 that were subjected to plasma-basedcleaning processes and plasma-based deposition processes, wherecolorizing UV radiation λ_(C)<300 nm is a byproduct, resulted in thepink colorization or hue.

Bleaching experiments were also performed. The bleaching experimentsshowed that bleaching of a colorized glass sample 10 can be obtainedwith UV radiation in the wavelength range from 254 nm to 360 nm and atintensity levels of I_(B) from 10 mW/cm² to 20 mW/cm². As noted above,in an example bleaching UV radiation 30 can have an intensity I_(B) inthe range 5 mW/cm²≦I_(B)≦100 mW/cm².

One explanation for the different wavelength ranges for colorization andbleaching is that a two-photon absorption process is taking place tocreate the Ti⁺³ color centers in colorized glass sample 10. This meansthat the intensity and energy of the colorizing UV radiation 30 must besufficient to produce a two-photon absorption to excite electrons fromthe valence band into the conduction band. The electrons then gettrapped by Ti⁺⁴ ions to form Ti⁺³ ions, which becomes color centers. TheTi⁺³ ions have additional excited states that lie above the conductionband.

In an example, the bleaching process involves irradiating colorizedglass sample 10 with bleaching UV radiation 30 to return glass sample 10to about its original absorbance by substantially removing the Ti⁺³color centers formed during the colorization process. This occurs whenthe electrons that formed the Ti⁺³ ions recombine with the trapped holeto form Ti⁺⁴ ions. The band of excited states is broad, whichcorresponds to the broad spectral region over which bleaching can occur.

To further define the spectral range of the colorization wavelengthλ_(C) of the colorization process, a high-power tripled YAG exposure ofan aluminophosphosilicate glass sample 10 was performed in an attempt tocause colorization. The exposure wavelength was λ=355 nm, whichcorresponds to an energy E=3.5 eV. No colorization was observed, whichsuggests the range the colorization wavelength λ_(C) is below 355 nm,such as 300 nm, which corresponds to an energy E of 4.13 eV.

Simple Model for Colorization and Bleaching

A simple kinetic equation for the rate of production of the Ti⁺³ colorcenters is given by

dT/dt=k·I ² _(C)(T ₀ −T)−q·[I _(B) +I _(C) ]·T  EQ. (1)

where T is the number of Ti⁺³ color centers, I_(C) is the intensity ofthe colorization UV radiation 20 at the colorization wavelength λ_(C),I_(B) is the intensity of the bleaching UV radiation 30 at the bleachingwavelength λ_(B), T₀ is the total concentration of Ti in glass sample10, and k and q are the colorization-rate constant and bleaching-rateconstant, respectively.

While the colorization and bleaching intensities I_(C) and I_(B) can bespectrally broad, for simplicity they are represented as a single valueat a single wavelength. Equation (1) does not account for anycompetitive absorption at the colorization wavelength, or any changewith time of such an absorption process.

The solution to Equation (1) is given by:

T/T ₀=(C/a)·[1−e ^(−aτ)]  EQ. (2)

where τ=I_(B)·q·t, C=(k/q)(I_(C)/I_(B))·I_(C) anda=(k/q)(I_(C)/I_(B))·I_(C)+(I_(C)/I_(B))+1. The steady state value forT/T₀=C/a. Equation (2) can be simplified by assuming that I_(C)>>I_(B),so that the resulting Equation (3) set forth below has T/T0 depending onthe ratio of the colorization-rate constant k to the bleaching-rateconstant q and the colorization intensity I_(C).

T/T ₀=(k/q)I _(C)/[(k/q)I _(C)+1]  EQ. (3)

Because two-photon absorption was used to describe the colorizationprocess that gives rise to the Ti⁺³ color centers, it can be reasonablyassumed that its cross-section is much smaller than that for a singlephoton event, so that k<<q. This assumption makes the term k·I_(C)/qsmall unless the colorization intensity I_(C) is extremely high, whichis unlikely. So a reasonable solution to Equation (3) assumes that theTi⁺³ concentration is relatively small and linear with colorizationintensity I_(C). This reduces Equation (3) to Equation (4):

T/T ₀ =Ti ⁺³ /Ti ⁺⁴ ≅k·I _(C) /q  EQ. (4)

FIG. 7 is a plot of the optical absorption A (mm⁻¹) as a function ofexposure time t in hours (hrs) for bleaching UV radiation 30 for anexample aluminophosphosilicate colorized glass sample 10 having the samecomposition as the glass sample mentioned above in connection with FIG.6. The bleaching UV radiation 30 had a bleaching wavelength λ_(B)=254 nmand an intensity I_(B)=5 mW/cm². From the plot of FIG. 7, an estimate ofthe bleaching-rate constant q can be obtained by taking a best-fit lineto the data, which is given by the slope/intercept form ofy=mx+b=−0.0135x+0.0473, with a “R-squared” fit of R²=0.9409. FromEquation (1), the slope m of the curve of FIG. 7 defines the parametera=I_(B)·q·t. Using the data in FIG. 7 to solve for q yields q=0.0027cm²/mW-hr.

Aspects of the disclosure include methods of processing glass sample 10to reduce or substantially eliminate UV-induced colorization. One suchmethod includes first irradiating glass sample 10 with colorizing UVradiation 20 having a colorizing wavelength of λ_(C)<300 nm to form acolorized glass sample having a pink hue. The method then includesirradiating the colorized glass sample 10 with bleaching UV radiation 30having a bleaching wavelength of λ_(B), wherein 248 nm≦λ_(B)≦365 nm, tosubstantially remove the pink hue.

Another aspect of the disclosure includes a method of processing glasssample 10, wherein the method includes irradiating the glass sample withcolorizing UV radiation 20 having a colorizing wavelength λ_(C)<300 nmto form a colorized glass sample having a pink hue due to the formationof an amount of Ti⁺³ color centers in the glass sample. The method thenincludes irradiating the colorized glass sample 10 with bleaching UVradiation 30 having a bleaching intensity I_(B) and a bleachingwavelength of λ_(B), wherein 248 nm≦λ_(B)≦365 nm, wherein the bleachingintensity I_(B) is sufficient to substantially remove the pink hue bysubstantially reducing the amount of Ti⁺³ color centers in the glasssample.

Another aspect of the disclosure includes a method of bleaching acolorized glass sample that has a pink hue due to the formation of Ti⁺³color centers. The method include irradiating the colorized glass samplewith bleaching UV radiation 30 that has a bleaching wavelength λ_(B),wherein 248 nm≦λ_(B)≦365 nm. The method also includes carrying out saidirradiating for an exposure time t in the range 0.5 hr≦t≦5 hrs tosubstantially remove the pink hue.

The irradiating with the bleaching UV radiation 30 can be a blanketirradiation, a scanned irradiation or a stepped irradiation. Thebleaching UV radiation 30 can be CW or pulsed.

It will be apparent to those skilled in the art that variousmodifications to the preferred embodiments of the disclosure asdescribed herein can be made without departing from the spirit or scopeof the disclosure as defined in the appended claims. Thus, thedisclosure covers the modifications and variations provided they comewithin the scope of the appended claims and the equivalents thereto.

1.-16. (canceled)
 17. A method of bleaching a colorized glass samplethat has a pink hue due to the formation of Ti⁺³ color centers,comprising: irradiating the colorized glass sample with bleaching UVradiation that has a bleaching wavelength λ_(B), wherein 248nm≦λ_(B)≦365 nm; and carrying out said irradiating for an exposure timet in the range 0.5 hr≦t≦5 hrs to substantially remove the pink hue. 18.The method according to claim 17, wherein the glass sample comprises analuminophosphosilicate glass.
 19. The method according to claim 17,wherein the Ti⁺³ color centers form from an amount T of titanium in theglass sample, wherein 20 ppm/mole≦T≦1000 ppm/mole.
 20. The methodaccording to claim 17, wherein the glass sample defines a glass sheet.21. The method according to claim 17, wherein the glass sample includestitanium in trace amounts T, wherein 20 ppm/mole≦T≦1000 ppm/mole. 22.The method according to claim 17, wherein the bleaching UV radiation hasa bleaching intensity I_(B) in the range 5 mW/cm² to 100 mW/cm².